First neighbouring planets that are both life-friendly

Twelve hundred light years away, two warmish Earth-size planets are circling the same dim red star. Travelling in adjacent orbits, both worlds may be covered in liquid water, making this the first known solar system outside our own to have more than one life-friendly planet.

Though there's a huge distinction between being life-friendly and actually hosting life, the existence of side-by-side habitable worlds raises the possibility of aliens who are one step ahead of earthlings and have already discovered life outside their own planet.

"These are very exciting planets," says Lisa Kaltenegger of Harvard University. Together with scientists working with on NASA's Kepler space telescope, she announced the discoveries at NASA's Ames Research Center in Moffett Field, California today.

Known as Kepler 62e and Kepler 62f, the worlds are both about 1½ times the size of Earth, which means they are most likely rocky. Two Earth diameters is the cut-off below which planets should be solid, says Kaltenegger. Larger than that and planets may be mini-Neptunes – small, gaseous worlds with no defined solid surface.

Goldilocks worlds

The Kepler team, who use dips in starlight to deduce the presence of planets passing in front of stars, actually saw five planets – b, c, d, e and f - around the star in question, Kepler 62. But only e and f orbit in the habitable zone (see image), the region where temperatures should be neither too hot nor too cold, but just right for liquid water to exist.

So could there be life on both planets, maybe even of the intelligent variety? It's too early to say, but there is certainly a chance, according to Kaltenegger. "You've got two solid planets in the habitable zone of their star. We don't know if life evolves everywhere, but if it does, these two planets provide great candidates for life," she says.

Theoretical models of these so-called Goldilocks planets suggest they may be covered entirely with water, which might make for life quite different to that on Earth – with no easy access to metals, electricity or fire. But Kaltenegger says it is also possible that they have oceans and continents, like Earth.

That prospect raises some fascinating possibilities – including that populations of aliens from each planet may already have communicated, even met, each other. "Imagine looking through a telescope to see another world with life just a few million miles from your own, or having the capability to travel between them on regular basis," says Dimitar Sasselov, also at Harvard. "I can't think of a more powerful motivation to become a space-faring society."

Sunlike star

If they exist, those denizens experience quite a different sun to Earthlings: Kepler 62 is smaller and cooler than our sun, so would glow red. But today the Kepler team also announced that a star very similar to ours has a potentially life-friendly planet, Kepler 69c – about 1.7 times the size of Earth.

"This is the first time we have found a potentially rocky exoplanet in the habitable zone around a star like our sun," says Kepler scientist Thomas Barclay of the Bay Area Environmental Research Institute in Sonoma, California.

Kepler 69c orbits its star at about the same distance that Venus orbits the sun (see image). While Venus is on the inner edge of our solar system's habitable zone, its thick atmosphere creates a greenhouse effect that keeps it far too warm for life as we know it. Such a fate is also possible for Kepler 69c, says Barclay – though it's too soon to say whether its atmosphere is Earth or Venus-like.

Although none of the new planets are Earth's exact twin, they are similar enough to bolster the idea that Earth-like worlds are relatively common around other stars, says Barclay.

"The Earth is starting to look less special as time goes on," he says.

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The Kepler 62 system: homes away from home (Image: JPL-Caltech/Ames/NASA)